This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-113079 filed on Apr. 11, 2001.
This invention relates to a pressure sensor for outputting a signal corresponding to an applied pressure by means of a piezo-resistance effect, made by bonding a sensor element to a metal stem.
Pressure sensors of this kind have been proposed which, to detect high pressures, are made by bonding a sensor element (sensor chip) made from a semiconductor to a metal stem with a low melting point glass, as disclosed in U.S. Pat. No. 4,840,067.
However, in this pressure sensor, because a low melting point glass is used to bond the sensor chip to the metal stem, heating to a temperature (for example about 480xc2x0 C.) is necessary. Therefore, this bonding is not suitable for sensor chips which are vulnerable to heat (for example chips with integrated circuit parts).
Resin adhesives are considered to be used for bonding sensor elements to metal stems, as a bonding method which can be implemented at relatively low temperatures. However, when low melting point glass is simply replaced with a resin adhesive, as a result of creep progressing in the adhesive resin with temperature changes, stresses act on the sensor chip, and the output of the sensor fluctuates with time.
It is therefore an object of the present invention to reduce fluctuations in sensor output caused by temperature changes in a pressure sensor made by bonding a sensor element to a metal stem with a resin adhesive.
The present invention is based on a supposition that, if the progressing of creep which accompanies temperature changes in the resin adhesive were to be slowed, then fluctuation of the sensor output could be reduced. It is known that, in a resin, the relationship between the creep rate CR and the stress "sgr" acting on the resin is given by the following expression.
CR=Axc3x97"sgr"B (where A and B are constants) 
According to the present invention, a pressure sensor has a sensor element operable based on the piezo-resistance effect and a metal stem bonded to this sensor element and supporting the sensor element. The sensor element and the metal stem are bonded with a resin adhesive. This resin adhesive has a creep characteristic such that the constant B in the above expression is not greater than 3.5, preferably to be less than 3.0. Thus, sensor output fluctuation could be reduced to a practically tolerable level. Further preferably, the resin adhesive may be made of an imide resin.